The invention relates to a ground fault monitoring device for an electrical power distribution system comprising means for measuring, filtering, sampling and analog-to-digital conversion of the residual voltage of an electrical power distribution system, means for measuring, filtering, sampling and analog-to-digital conversion of the residual current, means for synchronizing sampling of the residual voltage and current, a digital processing circuit connected to conversion outputs and supplying a fault signal.
Monitoring devices for protection against ground faults are designed to actuate indicators or to bring about opening of the line on which a fault has been detected. These devices can be found on several types of power systems which are differentiated by the voltage level (low voltage, medium voltage, high and extra high voltage) and type of common conductor (neutral) earthing, notably: isolated neutral, earthed neutral, impedant neutral or compensated neutral. A ground fault is defined by a short-circuit between one or more active lines of the power system (phases) and earth. As a general rule, ground fault current detection is performed by measuring a residual current in the phases of the line to be monitored. But this measurement is not always sufficient as the lines of great length have with earth line capacitances which generate reactive ground fault currents. The residual voltage of the power system therefore has to be measured to enable the active part and the reactive part of the ground fault current to be extracted.
State-of-the-art monitoring devices for protection against ground faults are based on detection of the active residual power or of the active residual current. Their operating mode is adapted to steady-state operating conditions and does not enable fault detection in transient operating conditions. Certain complementary solutions have also been proposed for fault detection in transient operating conditions, using instantaneous signals. This processing is very complex, and therefore costly.